This invention relates to an electrical device a property of which is a function of its temperature. In particular it relates to the use of such a device as a heater, particularly one that is self-regulating with respect to temperature. The device is particularly useful in the telecommunications, power, electrical and process industries, for example for activating sealing materials or causing heat-recovery of cable accessories or for trace heating.
The invention will be described primarily in terms of conductive polymer compositions, especially those containing a conductive filler rendering a base polymer conductive, and in terms of devices exhibiting positive temperature coefficient of resistance (PTC) behaviour. However it is to be understood that the invention is applicable to other materials, such as ceramics and inherently conductive materials, and to materials exhibiting other types of dependence on temperature, for example a negative temperature coefficient of resistance (NTC).
In designing devices having some property that is a function of temperature we have noticed that difficulties may arise in achieving the desired relationship between temperature and the variable concerned. This difficulty tends to occur particularly if one attempts to rely solely on the variation of some materials property with temperature. A PTC conductive polymer device will be described to illustrate this problem, and to explain a prior art solution.
PTC conductive polymer devices have been used for many years as heaters, particularly in tape form for trace heating: they may be wound around pipework in the process industry to maintain a certain temperature thus ensuring correct flow rates and preventing salting out. PTC materials automatically self-regulate their temperature thus avoiding the need for thermostats or fuses etc.
The distinguishing characteristic of PTC materials, therefore, is that on attaining a certain temperature, a substantial rise in resistance occurs. They exhibit a more or less sharp rise in resistance within a narrow temperature range, but below that temperature range exhibit only relatively small changes in resistance with temperature. The temperature or range at which the resistance commences to rise sharply is often designated the switching or anomaly temperature (T.sub.s) since on reaching that temperature the heater exhibits an anomalous change in resistance, and for practical purposes switches off.
A widely used PTC material is doped barium titanate which has been used for self-regulating ceramic heaters employed in such applications as food warming trays and other small portable appliances. Polymeric PTC materials are also known, as they generally comprise one or more conductive fillers such as carbon black or powdered metal dispersed in a crystalline thermoplastic polymer. PTC compositions prepared from highly crystalline polymers generally exhibit a steep rise in resistance commencing a few degrees below their crystalline melting points, similar to the behaviour of their ceramic counterparts at the curie temperature. PTC compositions derived from polymers and copolymers of lower crystallinity, for example less than about 50% exhibit somewhat less steep increases in resistance, which increase commences at a less well defined temperature in a range often considerably below the polymer's crystalline melting points. In the extreme case some polymers of low crystallinity yield resistance temperature curves which are more or less convex upwards. In addition to the effect of the polymeric material, one should also consider the type and amount of conductive filler. Particularly in the case of carbon black, one should consider its particle size, shape, surface characteristics, tendency to agglomerate and the shape of the agglomerates (i.e. its tendency to structure).
In the case of most prior art PTC materials, one sees a relatively constant wattage (CW) output below the switching temperature of range (T.sub.s). The resistance (R) at temperatures below T.sub.s is relatively low and constant and thus the current flow is relatively high for any given applied voltage E (I=E/R). The power generated by this current flow may be dissipated as Joule heat, i.e. heat generated by electrical resistance. This power, which is given by E.sup.2 /R, will heat the PTC material. The resistance stays at this relatively low level until about T.sub.s, at which point a rapid rise in resistance occurs. With the rise in resistance there is a concomitant decrease in power, thereby limiting the amount of heat generated so when T.sub.s is reached heating is essentially stopped. Then upon a lowering of the temperature of the device below T.sub.s by dissipation of heat to the surroundings, the resistance drops, thereby increasing the power output.
At a steady state, therefore, the heat generated will essentially balance the heat dissipated. Heat output is automatically regulated to maintain a temperature of about T.sub.s.